Well, this is a big issue - no matter what you do, there are still spam comments that even the best plugins cannot detect. I recommend posting comments in old posts, visitors would like to put valuable comments in new posts and you can drive targeted web traffic to your website through old post.

Unfortunately, after extensive research, I cannot find any plugins that can erase existing spam. I found a plugin that reviews existing comments but has not been updated since 2009. Some tips I suggest: